Cerium-containing oxide nanoparticles have many current industrial uses, along with many emerging technical applications. They are well known as important components, for example, in three-way automotive exhaust catalysts, automotive fuel borne catalysts, water gas shift reaction catalysts, polishing and planarization agents, solid oxide fuel cells, hybrid solar cells and ultra-violet sun blockers. There are many synthetic processes for the production of metal oxides, including aqueous and hydrothermal precipitation, spray precipitation, combustion, plasma deposition and electrochemical techniques, among others. While water may be utilized as a solvent in each of these synthetic processes, aqueous reaction chemistries are particularly favored in manufacturing processes where high material through-put is desired. However, conventional aqueous processes—precipitation in particular—are costly as they involve multiple steps that are often time and energy consuming, as well as equipment intensive.
Conventional large-scale metal oxide manufacturing processes can typically be divided into three stages: aqueous precipitation of precursor compounds, calcination to promote chemical reaction and to enhance crystallinity, followed by final particle size adjustment. In more detail, aqueous precipitation includes the initial steps of reactant dispersal, reactant delivery, particle precipitation, isolation, washing, drying, and optional impregnation with other metal ions; calcination involves heating to 400-1000° C. for several hours; followed by grinding, milling or classification to adjust the final particle size, among other steps.
One approach to reduce the number of steps in the aqueous precipitation stage is to employ methods that produce a stable aqueous dispersion (suspension) of the final particles directly from the initial reactants, thereby avoiding the time, cost and potential contamination inherent in the particle precipitation, isolation, and drying steps. Moreover, if the particles produced in such a direct method are sufficiently pure, wherein the chemical composition of the particles is as desired, and the particles are sufficiently crystalline, then the calcination step may also be eliminated. In addition, if the particle size and size distribution produced by such a direct method are substantially as desired, then the grinding, milling and classification steps may also be eliminated. Direct methods to produce aqueous dispersions (suspensions) of crystalline cerium-containing nanoparticles without the use of precipitation, isolation, drying, calcination, grinding, milling or classification steps, and the like, are described in commonly assigned U.S. Provisional Patent Application Ser. No. 12/779,602, CERIUM-CONTAINING NANOPARTICLES, filed May 13, 2010 by A. G. DiFrancesco et al., wherein stable aqueous dispersions of crystalline cerium-containing nanoparticles in a size range, for example, of 1-5 nanometers are described.
While substantial progress has been made in eliminating manufacturing steps from the synthetic process by which stable aqueous dispersions of metal oxide nanoparticles are prepared, use of these nanoparticles in applications such as fuel-borne combustion catalysts requires that dispersions of these nanoparticles also exhibit stability in the fuel, such that the nanoparticles remain suspended and do not settle out. Thus, these particles, although readily formed and suspended in a highly polar aqueous phase, must then be transferred to a substantially non-polar phase, a process known as solvent shifting. This problem is conventionally addressed by the use of particle stabilizers. However, most particle stabilizers used to prevent particle agglomeration in an aqueous environment are ill-suited to the task of stabilization in a non-polar environment. When placed in a non-polar solvent, such particles tend to immediately agglomerate and, consequently, lose some, if not all, of their desirable particulate properties. Changing stabilizers can involve a difficult displacement reaction or separate, tedious isolation and re-dispersal methods such as, for example, precipitation and subsequent re-dispersal with a new stabilizer using, for instance, a ball milling process, which can take several days and tends to produce polydisperse size frequency distributions.
One approach to simplify the solvent shifting process employs diafiltration methods and glycol ether solvents of a polarity intermediate between that of water and those of non-polar hydrocarbons, which are used to reduce the polarity of a cerium-containing nanoparticle dispersion, as disclosed in commonly assigned U.S. patent application Ser. No. 12/549,776, PROCESS FOR SOLVENT SHIFTING A NANOPARTICLE DISPERSION, filed Aug. 28, 2009. Diafiltration, sometimes referred to as cross-flow microfiltration, is a tangential flow filtration method that employs a bulk solvent flow tangential to a semi-permeable membrane. However, drawbacks of diafiltration methods include the following: relatively slow filtration rates (i.e. time consuming), substantial financial investment in equipment (e.g. pumps and microfilters), and production of a relatively large amount (e.g. several turnover volumes) of waste solvent.
To date, some progress has been achieved in reducing the cost of producing, and solvent shifting, aqueous dispersions of cerium-containing nanoparticles. However, further improvements in manufacturing efficiency are desired, particularly in the case of nanoparticle dispersions used as fuel-borne combustion catalysts that require dispersion stability in either a non-polar solvent carrier or in the fuel.